Field of the Invention
The present invention concerns a digital video system for assisting automobile traffic wherein traffic flow and traffic build-up signals are transmitted along a radio-frequency channel by a transmitter unit to receiver units placed in motor vehicles and are displayed on screens that can be seen by the drivers.
German Pat. No. 2 115 547, filed Mar. 31, 1971, has disclosed a process for transmitting through television channels provisional graphical information traced on a permanent map or plan without transmitting the permanent map or plan itself. Two identical maps or plans are located at the originating and terminating stations of the television channel and the map or plan at the originating station is coveed with a polarizer parallel sided shet. The provisional information is traced on this sheet and a transmit camera, the objection of which is covered by a polarizer member whose polarization direction is perpendicular to the polarization direction of the polarizer sheet scans the map or plan together with the provisional information traced thereon. Only the provisional information is transmitted and at the terminating station it is combined with the permanent information locally provided by a receive camera scanning the map or plan at the terminating station.
This prior art process presents several drawbacks when used for providing traffic information to car drivers. First, transmission is achieved through television channels which necessitates the use of ultra high frequency carriers and the use of cameras, impractical in ordinary cars. Secondly, the traffic state announcements are made irregularly only when the traffic undergoes at least one substantial change. Thus, the traffic information state at any instant comprises a part changed and a part unchanged from the preceding announcement. The prior art does not provide any means for distinguishing these two parts of the traffic information and for synthetizing the same.